In the recovery of oil from oil-containing formations, it is usually possible to recover only minor portions of the original oil in-place by the so called primary recovery methods which utilize only the natural forces present in the formation. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean formations. These techniques include thermal recovery methods, e.g., steam-flooding, water flooding, carbon dioxide and miscible flooding.
In an oil reservoir, the rocks may be capable of exhibiting high vertical and horizontal absolute permeability, although the mobility (i.e., permeability divided by viscosity) of fluid within the reservoir may be low and/or in-homogenous due to variations in reservoir lithology and fluid distribution within some or all of the pore spaces. In such a relatively non-stratified reservoir, fluids tend to become segregated by density. When a stratification of mobility is encountered it is usually due to a distribution of the oil, for example, in a layer above the water-rich, or below a gas-rich, portion of the reservoir.
When steam is flowed through such a reservoir at a pressure less than the fracturing pressure, steam tends to form a finger or channel that overrides a significant portion of the reservoir due to gravity segregation. Such a channel usually rides or runs along the upper portion of the reservoir, due to the effects of gravity. However, where an oil-rich layer overrides a water-rich layer, a steam channel may run along the upper portion of the water layer, near its junction with the oil layer. As is known to those skilled in the art, in such reservoirs, a steam channel can be relatively quickly formed and extended between horizontally separated injection and production locations for example, by injecting steam at a pressure near or less than the fracturing pressure while producing fluids as rapidly as possible.
In addition to the problems of channeling, diatomaceous intervals of a formation present additional problems. As is known, diatomaceous intervals comprise soft bulky solid material (88% silica) that is composed of skeletons of small prehistoric aquatic plants related to algae (diatoms). This type of formation is characterized by high porosity and low permeability. Steam-flooding is the most viable method to be utilized, from the standpoint of varying oil composition and viscosity in such formations or reservoirs. However, the nature of the formation with the significant silicon content renders a steam-flooding operation ineffective. This occurs because silicon salts in the formation become more soluble in a liquid component of steam at high temperatures and pH's. Continued steam-flooding in such a formation results in a dissolution of the silicon salts which can lead to formation caving near the wellbore. Additionally, it may lead to considerable formation damage due to the migration of fine (small sized, less than 30 microns) sand particles resulting in further permeability reduction away from the wellbore.
Therefore, what is needed is a method of steam-flooding in a diatomaceous or siliceous formation which will not remove silicon salts and avoid caving near the wellbore and permeability reduction away from the wellbore.